A terminal at Heathrow Airport was evacuated as fire crews were called to a "possible hazardous materials incident".
Police closed Terminal 4 while emergency services dealt with the incident.
The Metropolitan Police said around 20 people were injured but confirmed no "adverse substance was found".
London Fire Brigade has since confirmed that it was "in the process of stepping down".
A Met Police spokeswoman said: "Specialist officers attended the scene alongside the London Fire Brigade and London Ambulance Service and conducted a thorough search of the area.
"No trace of any adverse substance was found.
"Around twenty people reported an injury. None were deemed to be life-threatening or life-changing. Enquiries are ongoing."
The London Fire Brigade said part of the airport had been evacuated "as a precaution".
A member of airport staff was recorded giving updates to passengers
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London Ambulance Service said it was called to Terminal 4 at 4.50pm.
It confirmed ambulance crews treated 21 patients, with one taken to hospital while the others were discharged at the scene.
A spokesman for London Fire Brigade said: "Firefighters are responding to a possible hazardous materials incident at Heathrow Airport.
"Specialist crews have been deployed to carry out an assessment of the scene, and part of the airport has been evacuated as a precaution whilst firefighters respond."
Airport passengers were evacuated from Terminal 4 as a result of the incident
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Trains were unable to call at Heathrow Terminal 4 for around two hours while the incident was ongoing.
Footage posted to social media shows passengers standing outside the airport with their luggage as they await updates on when they can travel.
Some could be seen wrapped in foil blankets.
A member of staff with a megaphone could be heard in one clip saying: "We do apologise for the inconvenience, we do have a specialist team on site.
"This is an ongoing incident we have got going on in Terminal Four at the moment.
"I can't give any more of an update at the moment as to when you can come back in the building."
According to the airport’s website, flights from Terminal 4 are still scheduled to depart on time, with only one delay to a Saudia airline flight to Jeddah in Saudi Arabia currently expected.
Heathrow Airport, which is the busiest in Britain, confirmed that the affected terminal was reopening.
Fire crews rushed to the scene at around 5pm
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An airport spokeswoman said just after 8pm: “Emergency services have confirmed Terminal 4 is safe to reopen and we are doing everything we can to ensure all flights to depart as planned today.
"We are very sorry for the disruption caused, the safety and security of our passengers and colleagues is our number one priority.
"We encourage passengers to check with their airline for the latest information about their flight this evening and our colleagues will be on hand into the night to assist.”
"The exact cause remains under investigation at this time", London Fire Brigade posted to X.